1. Technical Field of the Invention
The present invention relates to a variable transmission ratio steering apparatus for a vehicle which has the capability of varying the transmission ratio of a steering angle for a steering wheel of the vehicle.
2. Description of the Related Art
Existing variable transmission ratio steering apparatuses for vehicles are generally configured with an input shaft, an output shaft, an actuator, a lock pin, a solenoid coil, and a controller. Such a steering apparatus is proposed, for example, in Japanese Patent Application Laid-Open No. 2001-278087.
In the apparatus, the input shaft is coupled to a steering wheel of the vehicle, and the output shaft is operatively linked to steerable road wheels of the vehicle. The actuator has an electric motor disposed between the input shaft and the output shaft. The electric motor is so configured to vary a transmission ratio between the rotation angle of the output shaft and the rotation angle of the input shaft. The lock pin is provided to connect the input shaft to the output shaft, and the solenoid coil is configured to actuate the lock pin to disconnect the input shaft from the output shaft. The controller controls the driving of the electric motor and the solenoid coil.
In particular, the controller controls the rotation angle of a rotor of the electric motor in response to the vehicle speed. When the vehicle speed is low, the controller drives the electric motor at a high rotation speed, so that the steerable road wheels of the vehicle can be largely steered with a small steering wheel manipulation.
However, in such a configuration, when the rotation speed of the rotor of the electric motor exceeds a certain value, audible operation noises arise from the electric motor, thus causing an uncomfortable feeling for the occupants of the vehicle.